MyotonPRO Is Not Comparable to Shear Wave Elastography in the Measurement of Rectus Femoris Muscle Stiffness due to Interference of Subcutaneous Adipose Tissue.

Tissue biomechanical properties are important for optimizing musculoskeletal performance and reducing injury risk. The MyotonPRO was developed as a simple, portable, noninvasive, and highly reliable method for the estimation of muscle stiffness. It has not, however, been comprehensively validated against shear wave elastography (SWE), a validated ultrasound-based method for assessing tissue stiffness. To assess this, 20 participants completed two visits in which rectus femoris (RF) stiffness was measured twice with the MyotonPRO device and SWE at three distinct muscle regions (proximal, medial, and distal), three muscle lengths (relaxed-REL, neutral-NEU, and passively stretched-PAST), and four depths (skin-SKIN, fascia-FAS, superficial muscle-SUP, and deep muscle-DEEP). Additionally, subcutaneous adipose tissue thickness (SAT) was recorded at each location under every condition using B-mode ultrasound imaging. When pooling the regions in REL, the MyotonPRO exhibited a weak significant negative correlation with SWE in DEEP (r = -0.27, p = 0.020), but not at SKIN, FAS, or SUP. In NEU and PAST, stiffness estimated by the MyotonPRO had a moderate positive relationship with SWE in the SKIN (r = 0.66, p < 0.001; r = 0.60, p < 0.001), but weaker relationships with deeper tissue (FAS, r = 0.26, p = 0.023; r = 0.20, p = 0.069; SUP, r = 0.24, p = 0.036; r = 0.26, p = 0.023; DEEP, r = 0.315, p = 0.008; r = 0.454, p < 0.001, respectively). Stiffness measured by the MyotonPRO exhibited significant moderate-to-strong negative correlations with SAT in every region under every condition, except for medially in REL. These results suggest that the two methods cannot be used interchangeably to estimate the stiffness of the RF muscle.